Electrical Services — Part 1

Feb 1, 2009 12:00 PM, By Mike Holt, NEC Consultant

Size the neutral service-drop conductor to carry the maximum unbalanced load, per 220.61, but don't size it smaller than required by 250.24(C) and Table 250.66. It can't be smaller than that, because it must have sufficiently low impedance and current-carrying capacity to safely carry fault current and thus facilitate operation of an overcurrent device.

See if you can correctly answer this question: What size neutral conductor in a service drop do you need for a structure with a 400A service supplied with 500kcmil conductors if the maximum line-to-neutral load is no more than 100A?

According to Table 310.16, 3 AWG rated 100A at 75°C [110.14(C)] is sufficient to carry 100A of neutral current. However, the service neutral conductor can't be sized smaller than 1/0 AWG (per Table 250.66, based on the area of the ungrounded service conductors) [250.24(C)]. Therefore, the correct answer is 1/0 AWG.

Service-drop conductor vertical clearances

Service-drop conductor spans must maintain a minimum of 8 ft above a roof surface, for a minimum distance of 3 ft in all directions from the edge of the roof [230.24(A)]. There are exceptions, however. For example, if the slope of the roof exceeds 4 in. of vertical rise for every 12 in. of horizontal run, you can reduce the clearances of 120/208V or 120/240V conductors to 3 ft over the roof.

Overhead conductor spans must maintain the following vertical clearances:

10 ft above finished grade, sidewalks, or platforms or projections from which they might be accessible to pedestrians for 120/208V or 120/240V circuits.
12 ft above residential property and driveways, and those commercial areas not subject to truck traffic for 120/208V, 120/240V, or 277/480V circuits.
18 ft over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles, such as those used for cultivation, grazing, forestry, and orchards.

If overhead service conductors are not under the exclusive control of the electric utility [90.2(B)(5)] but are located above pools, outdoor spas, outdoor hot tubs, diving structures, observation stands, towers, or platforms, then install them per the clearance requirements of 680.8.

Attachment

The point of attachment for service-drop conductors must be at least 10 ft above the finished grade. Choose a location that allows you to maintain the minimum service conductor clearance required by 230.9 and 230.24 [230.26]. This means you don't just attach it at 10 ft and 1 in. and call it good. You might need to raise the point of attachment so you still comply with 230.9 and 230.24.

When attaching multiconductor service cables, use only fittings identified for use with service conductors [230.27]. Attach open service conductors to fittings identified for use with service conductors or to noncombustible, nonabsorbent insulators securely attached to the building or other structure.

Support

The service mast used as overhead conductor support must have adequate mechanical strength (or braces or guy wires to support it) to withstand the strain caused by the service-drop conductors [230.28]. Some local codes require a minimum 2-in. rigid metal conduit for the service mast. In addition, many electric utilities contain specific requirements for the installation of the service mast.

The only conductors you can attach to a service mast are the electric utility service-drop conductors. Don't attach cabling from other systems to it. NEC 810.12 and 820.44(C) also specify that aerial cables for radio, TV, or CATV must not be attached to the service mast, and 810.12 prohibits antennas from being attached to the service mast. In addition, 800.133(B) and 830.133(B) prohibit communications and broadband cables from being attached to raceways, including a service mast (Fig. 4 on page 28).

Underground service-lateral conductors

Underground service-lateral conductors under the exclusive control of an electric utility must comply with the National Electrical Safety Code (NESC), not the NEC [90.2(B)(5)]. Underground conductors that are not under the exclusive control of the electric utility must be installed in accordance with the NEC.

Service-lateral conductors must have adequate mechanical strength, and they must have sufficient ampacity to carry the load as calculated in Art. 220 [230.31(A)]. Service-lateral conductors must not be smaller than 8 AWG copper or 6 AWG aluminum [230.31(B)]. Exception: Service-lateral conductors can be as small as 12 AWG for limited-load installations.

Size the neutral service-lateral conductor to carry the maximum unbalanced load per 220.61. Don't size the service neutral conductor smaller than required by 250.24(C) and Table 250.66 [230.31(C)]. If you install service lateral conductors underground, make sure they have the minimum cover required by Table 300.5.

Now that we've reviewed the requirements for sizing and installing conductors that are coming from the service point, you've probably noticed Art. 230 presents these requirements in a logical sequence. You should be able to set your one-line diagram down right next to it and proceed straight through the requirements. In Part 2 of this article, you'll be able to make a similar observation.